1. Field of the Invention
The present invention relates to detecting and identifying metal targets in general and, more particularly, to an electromagnetic target discriminator (ETD) sensor system and method for detecting and identifying metal targets, such as unexploded ordnance, high metal content landmines and low metal content landmines (commonly referred to as plastic landmines) buried in the soil (or visually obscured) based on the electromagnetic response of the target to a time-domain wide bandwidth electromagnetic spectrum.
2. Description of the Related Art
With an estimated 100 million mines and countless millions of acres of land contaminated with unexploded ordnance (UXO) worldwide there is a need for sensor systems and methods that can detect and identify large and small metal objects buried in soil. In addition, during armed conflict, there is a need for mine detection and neutralization in real-time or near real-time.
A commonly used sensor for mine and UXO detection is the electromagnetic induction (EMI) metal detector. Conventional EMI metal detectors using either frequency-domain (FD) or time-domain (TD) eddy current methods can detect small metal targets (such as plastic-cased low-metal content mines) at shallow depths and large metal targets (such as metal-cased high-metal content mines and UXOs) at both shallow and deep depths under a wide range of environmental and soil conditions. However, metal non-mine (i.e., clutter) objects commonly found in the environment pose a major problem in identifying mines. That is because these clutter objects create false alarms when detected by a metal detector. For time-efficient and cost-effective land clearing, the detected metal targets must be classified as to their threat potential: mine, UXO or clutter. Preferably, these metal targets need to be classified in real-time or near real-time.
FIG. 1 shows a diagram of a conventional pulsed EMI metal detector and method of operation. A current loop transmitter 10 is placed in the vicinity of the buried metal target 12, and a steady current flows in the transmitter 10 for a sufficiently long time to allow turn-on transients in the soil (soil eddy currents) to dissipate. The transmitter loop current is then turned off. The transmitter current is typically a pulsed waveform. For example, a square-wave, triangle or sawtooth pulsed waveform, or a combination of different positive and negative current ramps.
According to Faraday's Law, the collapsing magnetic field induces an electromotive force (emf) in nearby conductors, such as the metal target 12. This emf causes eddy currents to flow in the conductor. Because there is no energy to sustain the eddy currents, they begin to decrease with a characteristic decay time that depends on the size, shape, and electrical and magnetic properties of the conductor. The decay currents generate a secondary magnetic field that is detected by a magnetic field receiver 14 located above the ground and coupled to the transmitter 10 via a data acquisition and control system 16.
Extensive theoretical and experimental research supports the concept of metal target classification using EMI techniques. In the time-domain for a pulsed transmitter current, the eddy current time decay response from metal target can be expressed as:                               V          ⁡                      (            t            )                          =                              δ            ⁡                          (              t              )                                -                                    ∑              i                        ⁢                          [                                                A                  i                                ⁢                exp                ⁢                                  {                                                            -                      t                                        /                                          τ                      i                                                        }                                            ]                                                          (        1        )            where t is time, V(t) is the induced voltage in the receiver coil, δ(t) is the delta function, Ai are object amplitude response coefficients, and τi are the object's time constants. Thus, the sensor response to a metal target is a sum of exponentials with a series of characteristic amplitudes, Ai, and time constants, τi. Equation (1) and its complimentary equation, i.e., in the frequency domain, form the theoretical basis of an EMI sensor's classification technique. If a metal target is shown to have a unique time decay response, a library of potential threat targets can be developed. When a metal target is encountered in the field, its time decay response can be compared to those in the library and, if a match is found, the metal target can be classified quickly. Equation (1) can also be applied to the response of the environment, particularly the soil or water that the metal target is buried in. It is noted that Equation (1) is slightly different if the transmitter current waveform is a ramp or other time-varying signal, but the general nature of the multiple exponential target response is the same.
The detection of buried objects having a metal content has been established abundantly in the scientific and engineering literature. There still, however, remains the problem of identifying buried objects using sensor systems and object identification methodology. The identification problem for buried metal targets is divided into two categories: (1) identification of medium to large metal content objects and (2) identification of low metal content objects.
Medium to large metal content mines and UXO objects have a unique eddy current time decay (or frequency) response characteristic that enables them to be discriminated from a wide variety of typical metal clutter in a variety of soil types. Medium to large metal content mines and UXO objects have many complex three-dimensional structural features that manifest themselves in different eddy current time decay or frequency spectrum characteristics. This complex eddy current decay response must be measured very accurately over many orders of magnitude in both time and amplitude (TD sensor) or frequency, amplitude and phase (FD sensor).
Typically, the medium to large metal content mines and UXO objects can be modeled spatially as simple point magnetic dipoles and their time or frequency decay response can be modeled with one or two time decay parameters. Generally, for medium and large metal objects, the soil does not adversely effect the time decay response measurements. This is due to the fact that the soil response is typically small and/or is confined to a small time decay region.
The identification of low metal content objects is more difficult, since they do not have complex structural features that manifest themselves in different eddy current time decay or frequency spectrum characteristics as compared to medium to large metal content objects which have a more complex spatial and time decay response. In addition, the time constant of the metal decay of low metal content objects is relatively fast, requiring a wide bandwidth EMI sensor system. Also, for most environments, the effect of the soil's TD (or FD) response must be taken into account when attempting to identify low metal content objects.
U.S. Pat. Nos. 5,963,035 and 6,104,193 describe low metal content mine detection and identification systems and approaches, but do not address the eddy current decay response of the soil (or the mine's environment in general). This is a major shortcoming since the library of target signatures must be accurately known in advance for high confidence object identification. If the soil modifies an object's decay response signature, the sensor system is probably not going to correctly identify the object using a target identification algorithm.
Once an object has been detected and target decay response data has been collected, a signal processing method is applied to the eddy current decay time response to identify the object. Prior art sensor systems do not optimize the sensor's data collection parameters for optimal target identification. These sensor parameters include transmitter field strength (i.e., current in transmitter coil), amplifier gain, sample rate of digitizer, and sample collection time. In addition, prior art sensor systems and signal processing approaches generally ignore the effects of soil on the measured target response, especially the effects of highly magnetic soils. If the soil response is not accounted for, the library of metal object signatures becomes less effective and less useful. Several prior art sensor systems also fail to take into account the non-uniform nature of their primary exciting magnetic field on the object's decay response characteristic.
Accordingly, a need exists for a sensor system for accurately measuring a metal target's decay response based on the physical parameters of the metal target and its environment and for identifying the metal target.